Sean MacStiofain, who has died aged 73, was one of the republican hardliners responsible for the emergence in 1969 of the Provisional IRA, of which he was the chief of staff during the violence of the early 1970s. He was responsible for a truce that made negotiation possible and led the group - including the young Gerry Adams and Martin McGuinness - which was flown to London in 1972 for talks with the then Northern Ireland secretary, William Whitelaw. But his hardline views, as well as the inevitability of power shifting north of the border, meant that he was ousted from the leadership in late 1973.

MacStiofain was an old-fashioned republican. He fell out with the pre-1969 Dublin leadership as it attempted to move into constitutional politics, and he fell out with the new leadership in Northern Ireland as it moved towards a mixture of constitutional pol itics and the politics of the gun and the bomb.

Born John Stephenson in Leytonstone, east London, his Belfast-born mother died when he was 10 but had told him: "I'm Irish, therefore you're Irish. You're half-Irish anyway. Don't forget it." He never did.

MacStiofain left school at 16 and joined the building trade. In 1945 he was conscripted into the RAF as a storeman and served in Jamaica. Back in London he learned Gaelic, married an Irish girl, involved himself in expatriate Irish politics and went with a like-minded group to Dublin in search of the IRA army council, which gave them permission to form a London unit.

His only IRA activity in England was in 1951, when the army council in Ireland began a border campaign and wanted arms. The senior republican Cathal Goulding was sent to England to join MacStiofain in raiding the armoury at Felsted school. They seized guns but were arrested, and MacStiofain served five years in prison.

After his release he moved to Ireland, working at first with and then against Goulding, who, as its chief of staff, was reshaping the IRA into a working-class constitutional movement. Goulding, a man with a witty tongue, was intent on abandoning the IRA policy of abstention from constitutional politics. As bitterness grew between them, Goulding said the humourless MacStiofain was trying to prove he was more Irish than anyone. MacStiofain never lost his cockney accent, but hated to be described as English or be called Stephenson.

In the late 1960s, as the Northern Ireland civil rights movement was enveloped in violence, MacStiofain and his Dublin allies Daithi O'Conaill and Ruairi O'Bradaigh, both old-fashioned, mass-going Catholic Republicans, searched for arms for the IRA in Derry and Belfast. Mac Stiofain's cross-border visits were in defiance of Goulding, who was insistent on maintaining a ban on IRA activity in the south. MacStiofain, an IRA intelligence officer since 1966, moved to Navan to coordinate border operations. From there, in August 1969, he made an abortive raid on the police station in Crossmaglen, the hardline IRA town in south Armagh.

At one stage MacStiofain advised contacts not to hand money to the IRA since Goulding planned to use half of its funds to pay organisers. This, MacStiofain said proudly, meant that after the split between Goulding's Official IRA and the Provisionals in 1971, he was able to find "generous" benefactors.

As violence flared in Northern Ireland, following attacks on civil rights marches and then internment in 1971 and Bloody Sunday in 1972, MacStiofain was central in the IRA army council. When Stormont rule was suspended by Edward Heath's government, MacStiofain claimed it as an IRA victory and offered a truce to the newly appointed Northern Ireland Secretary, William Whitelaw. Whitelaw was persuaded to consider the offer by John Hume and Paddy Devlin of the new constitutional nationalist party, the Social Democratic and Labour party. The ceasefire was delivered and MacStiofain, Seamus Twomey, O'Connaill, McGuinness and Adams - brought out of the Long Kesh internment camp - flew to London.

The Whitelaw negotiations were MacStiofain's high point. Before the London visit he fell out with O'Bradaigh, and after it the northern leaders became increasingly unhappy with him. McGuinness described him as an "egomaniac". South of the border, the Irish government, after the suspension of Stormont, began to arrest IRA leaders.

MacStiofain was arrested in 1973 and charged with IRA membership. He went on a 10-day hunger and thirst strike, which provoked rioting in Dublin. While in Mountjoy jail he was visited and given absolution by a former Archbishop of Dublin, but then persuaded to abandon the thirst strike. He maintained his hunger strike for 57 days - but then was ordered to end it by the army council.

His abandonment of the hunger strike was seen as weakness and effectively meant the end of his leadership, which fell first to Twomey and then to O'Bradaigh before, in the late 1970s, it passed to Adams and the young bloods of Northern Ireland.